1. Field of the Invention
The present invention relates to an arm member for use as, for example, an automobile suspension arm or an engine mount, and more particularly, to an arm member which is to be coupled with the other members and includes a coupling member with a bushing mounting portion, a yoke portion, a ball joint mounting portion, or the like.
The term "aluminum" used in this specification denotes aluminum and its alloys.
2. Description of the Related Art
FIGS. 8A and 8B illustrates an arm member 51 for use as an automobile suspension arm such as an upper arm, a lower arm, a trailing arm and a torsion arm.
The arm member 51 comprises a main body 52 and a pair of metallic coupling members 55 each connected to the respective end portion 53 of the main body 52. The main body 52 is made of a metallic cylindrical pipe having a predetermined length in order to lighten the arm member 51.
The coupling member 55 comprises a cylindrical bushing mounting portion 57 for mounting a columnar vibroisolating bushing 61 with a rubber elastic member, a columnar connecting portion 58 integrally outwardly protruded from an outer surface of the bushing mounting portion 57, and a columnar fitting protrusion 59 integrally outwardly protruded from the central portion of the end surface of the connecting portion 58. The fitting protrusion 59 has a diameter smaller than that of the connecting portion 58. The bushing mounting portion 57 has a circular bushing mounting opening 56. The vibroisolating bushing 61 is forcibly fitted in the bushing mounting opening 56 and secured therein.
In this arm member 51, as shown in FIG. 8B, the stepped end of the connecting portion 58 of the coupling member 55 is abutted to the end of the peripheral wall at the end portion 53 of the main body 52 to form an abutted portion 60. The coupling member 55 is connected to the main body 52 at the abutted portion 60. In this state, the fitting protrusion 59 of the coupling member 55 is tightly fitted in or forcibly fitted in the opening 52a of the end portion 53 of the main body 52. In FIG. 8B, the portion W' is a welded portion formed along the entire circumference of the abutted portion 60.
For joining the abutted portion 60, various joining methods can be adapted, such as a fusion welding method including a MIG welding, a TIG welding or a laser welding, or a friction agitation joining or welding method (hereinafter referred to as "friction agitation joining method) which is one type of solid-phase joining (welding) methods. In the arm member 51 shown in FIGS. 8A and 8B, the abutted portion 60 is joined, or welded, by a friction agitation joining method.
The friction agitation joining method will be explained as follows. In FIG. 8B, the reference numeral 70 denotes a joining tool for a friction agitation joining method. The joining tool 70 includes a columnar rotor 71 having a larger diameter and a pin-like probe 72 having a smaller diameter protruded from an end surface 71a of the rotor 71 along the axis of the rotor 71. The probe 72 rotates as the rotor 71 rotates. The rotor 71 and the probe 72 are harder than the main body 52 and the coupling member 55, and made of heat-resistant materials which can stand friction heat generated during the joining process. Convex portions (not shown) for agitating or stirring the softened materials are formed on the surface of the probe 72.
In the arm member 51, the main body 52 and the coupling member 55 are joined according to the following steps. First, while rotating the probe 72 on the rotor 71 of the joining tool 70, the rotating probe 72 is inserted into the abutted portion 60. Then, the rotating probe 72 is moved in the circumferential direction of the abutted portion 60 while softening and agitating the portion where the rotating prove 72 contacts by the friction heat. Thus, the abutted portion 60 is joined, or welded, along the entire circumference thereof. As a result, the coupling members 55 are coupled to the main body 52.
The arm member 51 is to be connected to the other member of an automobile (not shown) via the vibroisolating bushing 61 mounted in the bushing mounting opening 56 of the coupling member 55. Therefore, the main body 52 and the coupling members 55 must be joined firmly.
However, a conventional arm member, in which the main body and the coupling member are joined by a joining method such as a fusion welding (a MIG welding, a TIG welding, a laser welding, or the like) or a solid-phase welding method (a friction agitation joining method), may cause cracks at or approximate to the joined portion W' when a tensile load is imparted thereto.
Furthermore, in the arm member 51, since the main body 52 is made of a pipe member in order to lighten the weight of the arm member 51, thermal fatigue tends to occur at or approximate to the joined portion W'. The thermal fatigue may cause cracks at or approximate to the joined portion W', resulting in an easy-broken arm member 51.